An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly. The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. Any part of the pattern may be quoted to force the quoted portion to be matched as a string.
[[ expression ]] Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries.
When used with [[, the < and > operators sort lexicographically using the current locale.
See the description of the test builtin command (section SHELL BUILTIN COMMANDS below) for the handling of parameters (i.e. missing parameters).
When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching, as if the extglob shell option were enabled. The = operator is equivalent to ==. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 oth‐ erwise. Any part of the pattern may be quoted to force the quoted portion to be matched as a string.
An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the nocasematch shell option is enabled, the match is per‐ formed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force the quoted portion to be matched as a string. Bracket expressions in regular expressions must be treated carefully, since normal quoting characters lose their meanings between brackets. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched as a string. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression ) Returns the value of expression. This may be used to override the normal precedence of operators. ! expression True if expression is false. expression1 && expression2 True if both expression1 and expression2 are true. expression1 || expression2 True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.
string1 = string2 True if the strings are equal. = should be used with the test command for POSIX conformance. When used with the [[ command, this performs pattern matching as described above (Compound Commands).
raditional glob wildcards support a very narrow set of metacharacters — * is "anything", ? is an arbitrary single character; Bourne shell also supports [a-z123] for a single character out of a set of alternatives, and [!x-z789] any one except those listed.
Regex obviously is much richer, supporting repetitions, and (in ERE) alternation and specific numbers of repetitions. Perl-style regex further extends the formalism to the point where multiple books have been written, and more will be.
Basic regex is not altogether a lot more challenging to program than glob wildcards, and these days, a competent programmer would link to an existing library in either case, anyway.
Many simpler systems don't want to burden their users with the complexity of learning regular expressions — even basic wildcards are a challenge to explain to your average sales guy^W^W person who isn't a full-time computer user.
Regular expressions are used in commands / functions for pattern matching in text. For example in the pattern parameter of grep, or in programming languages.
Globbing is used by shells for matching file and directory names using wildcards. The capabilities of globbing depend on the shell. Bash, for example, supports wildcards like:
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